1. Field of the Invention
The present invention relates to a conversion apparatus which uses high-speed switching devices (abbreviated below as HSDs) possessing two functions, latch type and non-latch type functions.
2. Description of the Related Art
Power devices conventionally used in conversion apparatus for converting DC power to AC power, AC power to DC power, AC power to AC power with a different frequency or DC power to DC power with a different voltage include GTO thyristors (called GTOs below) and transistors. A GTO is a latch type device, which means that once the GTO is brought to an ON state by supplying a signal to its gate, anode current continues to flow even if the gate signal is made "0".
A transistor, on the other hand, conducts when a base signal is supplied but the collector current becomes "0" when the base signal is removed. A device such as this will be called a non-latch type device here.
GTOs are widely used as high-voltage, high-current semiconductor devices whose turn-on voltage is comparatively low. But since current filamentation occurs when they are switched OFF, their breakdown withstand capability is low and large snubber circuits are necessary. Also, their switching speed is not fast.
In contrast, an insulated gate bipolar transistor (called IGBT below) is not liable to break down, since there is no current filamentation, and its switching speed is fast, but it is not suitable for high-voltage applications. Progress has been made in the development of devices with fast switching speeds at high voltage, and one candidate is a device for which the concept is that when the device is in an ON state its structure is made into a latch type so as to reduce conduction losses and immediately before turn-off the device structure is changed over to a non-latch type so as to avoid current filamentation. A specific example is the device shown in Japanese Laid-open Patent Application No. 64-758.
An example of a typical conventional conversion apparatus using the latch type GTOs is shown in FIG. 8. In the figure, 1 is a DC power source, 2P and 2N are DC circuit breakers, 3 is a DC reactor, 4 is a DC capacitor, 5 is a reactor, 6 is a diode, 7 and 8 are DC buses, 11, 12, 13, 14, 15 and 16 are GTOs, 17, 18, 19, 20, 21 and 22 are feedback diodes and 25U, 25V and 25W are AC circuit breakers. This circuit is widely used and is well-known as an inverter and, although not shown, snubber circuits and gate circuits, etc. are needed, but a description of the operation of this circuit will be omitted here.
If, through incorrect operation, GTOs 11 and 12 are ON simultaneously, DC buses 7 and 8 are short-circuited, and so first the charge of DC capacitor 4 goes through reactor 5 into GTOs 11 and 12 and then current from DC power source 1 goes via DC reactor 3 and DC reactor 5 into GTOs 11 and 12.
Overcurrent protection of the devices in a circuit such as this is provided by including elements, usually reactors or fuses, which restrict the magnitude of current to values below the surge current withstand capability that can be withstood by the devices themselves. In, for example, the case of a GTO with which the turn-off current is 1000 A, the surge current withstand capability is 7000 A in the case of a sinusoidal half wave with a width of 10 ms. In FIG. 8, reactor 5 keeps short-circuit current to a value that is below the surge current withstand capability of GTOs 11 and 12, etc. and DC reactor 3 suppresses inflow of current from the DC power source 1 in the time up to when DC circuit breakers 2 are opened. Reactor 5 must be so designed that it does not become saturated by excessive current flowing in during this period. DC reactor 3 must be an element that is not saturated by the voltage-time product to which it is subjected during the short circuit period. Consequently, they are both made as large reactors. AC circuit breakers 25U, 25V and 25W shall be opened immediately when there is inflow of current from loads not shown or when fault circuits and loads are to be isolated.
On the other hand, if non-latch type devices, i.e., transistors, are used in the circuit of FIG. 8, since the devices themselves can act to suppress overcurrent through the device, the overcurrent can be stopped immediately by applying reverse bias to their bases if a short circuit occurs. Reactor 5 is not absolutely necessary but since it acts to suppress current if it is present, it allows a time delay up to the time of protective action following a short circuit. Since the current at the time of a short circuit is at most about double that of normal operation, even if reactor 5 is included it need only be a small reactor.
In conversion apparatus using HSDs which have been developed to provide conversion apparatuses which have the both advantages of using latch type devices and non-latch type devices, the respective advantages of the devices are displayed in normal operating states. In normal operation, turn-off action is performed after changeover to a non-latch type immediately before turn-off. But since in a steady ON state the devices are in a latch state, if, because of commutation failure, etc., a DC short circuit occurs and overcurrent flows, it is not possible to change the device state to a non-latch type. Consequently, it is not possible to effect turn-off at bases or gates as with transistors, and the present situation is that increases in current are suppressed by using large reactors as in the case when thyristors or GTOs are used.